PXR stimulates growth factor-mediated hepatocyte proliferation by cross-talk with the FOXO transcription factor

Abstract

Growth factor-mediated hepatocyte proliferation is crucial in liver regeneration and the recovery of liver function after injury. The nuclear receptor, pregnane X receptor (PXR), is a key transcription factor for the xenobiotic-induced expression of genes associated with various liver functions. Recently, we reported that PXR activation stimulates xenobiotic-induced hepatocyte proliferation. In the present study, we investigated whether PXR activation also stimulates growth factor-mediated hepatocyte proliferation. In G0 phase-synchronized, immortalized mouse hepatocytes, serum or epidermal growth factor treatment increased cell growth and this growth was augmented by the expression of mouse PXR and co-treatment with pregnenolone 16α-carbonitrile (PCN), a PXR ligand. In a liver regeneration model using carbon tetrachloride, PCN treatment enhanced the injury-induced increase in the number of Ki-67-positive nuclei as well as Ccna2 and Ccnb1 mRNA levels in wild-type (WT) but not Pxr-null mice. Chronological analysis of this model demonstrated that PCN treatment shifted the maximum cell proliferation to an earlier time point and increased the number of M-phase cells at those time points. In WT but not Pxr-null mice, PCN treatment reduced hepatic mRNA levels of genes involved in the suppression of G0/G1- and G1/S-phase transition, e.g. Rbl2, Cdkn1a and Cdkn1b. Analysis of the Rbl2 promoter revealed that PXR activation inhibited its Forkhead box O3 (FOXO3)-mediated transcription. Finally, the PXR-mediated enhancement of hepatocyte proliferation was inhibited by the expression of dominant active FOXO3 in vitro. The results of the present study suggest that PXR activation stimulates growth factor-mediated hepatocyte proliferation in mice, at least in part, through inhibiting FOXO3 from accelerating cell-cycle progression.

Acceleration of murine hepatocyte proliferation by imazalil through the activation of nuclear receptor PXR

“…Recently, we have demonstrated that PXR activation accelerates CAR-dependent as well as growth factor-mediated proliferation of murine hepatocytes by increasing the sensitivity to proliferation stimuli through the downregulation of cyclin-dependent kinase inhibitor proteins (Shizu et al, 2013(Shizu et al, , 2016. We thus co-treated mice with TCPOBOP and imazalil to verify whether imazalil treatment could accelerate hepatocyte proliferation induced by TCPOBOP through mPXR activation.…”

“…Total RNA isolation and cDNA synthesis were carried out as described previously (Shizu et al, 2016). Quantitative RT-PCR (qRT-PCR) was performed using GoTaq qPCR Master Mix (Promega) and primer pairs for genes of interest shown in Supplementary Table S1.…”

“…In addition to this well-known function, recent studies have expanded the roles of PXR, which include the regulation of glucose or lipid metabolisms in the liver (Sui et al, 2011;Chang and Waxman, 2006;Hukkanen, 2012;Gotoh and Negishi, 2015;Spruiell et al, 2014). In addition, we have recently demonstrated that PXR activation enhances hepatocyte proliferation mediated by CAR or peroxisome proliferator-activated receptor α (PPARα) activation: Although treatment of mice with pregnenolone 16α-carbonitrile (PCN), a PXR agonist, alone did not induce hepatocyte proliferation, co-treatment of mice with PCN and CAR or PPARα agonist showed intense hepatocyte proliferation compared to single treatment with CAR or PPARα agonist (Shizu et al, 2013), which probably results from the PXR-mediated down-regulation of cell cycle suppressor genes (Shizu et al, 2016). Because CAR-and PPARα-dependent hepatocyte proliferation is associated with liver cancer in rodents, the exposure to PXR activators might increase the risk of liver cancer, that is, PXR-activating capability of chemicals can be a potential risk of adverse effects.…”

Pregnane X Receptor and Cancer: Context-Specificity is Key

“…Finally, PXR has also been shown to play a role in promoting the growth and chemo-resistance of liver tumors (Braeuning et al, 2014; Rigalli et al, 2015). However, PXR over-expression portends a favorable prognosis in pancreatic adenocarcinoma (Koutsounas et al, 2015) but might aggravate liver tumors (Kodama and Negishi, 2011; Kodama et al, 2015; Shizu et al, 2015). PXR is also more recently implicated as a driver of DNA damage in cutaneous carcinogenesis (Elentner et al, 2015).…”

Regulation of PXR and CAR by protein-protein interaction and signaling crosstalk

“…PXR activation by PCNinhibits FoxO3-mediated transcriptional regulation of cell-cycle suppressor genes such as Rbl2 , thus promoting hepatocyte proliferation [97]. On the other hand, overexpressed FoxO3 inhibits the PXR-mediated enhancement of hepatocyte proliferation [97]. The regulation of hepatocyte proliferation by PXR has previously been reviewed by Pondugula et al [98].…”